Module « numpy »

Fonction diagonal - module numpy

Signature de la fonction diagonal

def diagonal(a, offset=0, axis1=0, axis2=1) 

Description

diagonal.__doc__

    Return specified diagonals.

    If `a` is 2-D, returns the diagonal of `a` with the given offset,
    i.e., the collection of elements of the form ``a[i, i+offset]``.  If
    `a` has more than two dimensions, then the axes specified by `axis1`
    and `axis2` are used to determine the 2-D sub-array whose diagonal is
    returned.  The shape of the resulting array can be determined by
    removing `axis1` and `axis2` and appending an index to the right equal
    to the size of the resulting diagonals.

    In versions of NumPy prior to 1.7, this function always returned a new,
    independent array containing a copy of the values in the diagonal.

    In NumPy 1.7 and 1.8, it continues to return a copy of the diagonal,
    but depending on this fact is deprecated. Writing to the resulting
    array continues to work as it used to, but a FutureWarning is issued.

    Starting in NumPy 1.9 it returns a read-only view on the original array.
    Attempting to write to the resulting array will produce an error.

    In some future release, it will return a read/write view and writing to
    the returned array will alter your original array.  The returned array
    will have the same type as the input array.

    If you don't write to the array returned by this function, then you can
    just ignore all of the above.

    If you depend on the current behavior, then we suggest copying the
    returned array explicitly, i.e., use ``np.diagonal(a).copy()`` instead
    of just ``np.diagonal(a)``. This will work with both past and future
    versions of NumPy.

    Parameters
    ----------
    a : array_like
        Array from which the diagonals are taken.
    offset : int, optional
        Offset of the diagonal from the main diagonal.  Can be positive or
        negative.  Defaults to main diagonal (0).
    axis1 : int, optional
        Axis to be used as the first axis of the 2-D sub-arrays from which
        the diagonals should be taken.  Defaults to first axis (0).
    axis2 : int, optional
        Axis to be used as the second axis of the 2-D sub-arrays from
        which the diagonals should be taken. Defaults to second axis (1).

    Returns
    -------
    array_of_diagonals : ndarray
        If `a` is 2-D, then a 1-D array containing the diagonal and of the
        same type as `a` is returned unless `a` is a `matrix`, in which case
        a 1-D array rather than a (2-D) `matrix` is returned in order to
        maintain backward compatibility.

        If ``a.ndim > 2``, then the dimensions specified by `axis1` and `axis2`
        are removed, and a new axis inserted at the end corresponding to the
        diagonal.

    Raises
    ------
    ValueError
        If the dimension of `a` is less than 2.

    See Also
    --------
    diag : MATLAB work-a-like for 1-D and 2-D arrays.
    diagflat : Create diagonal arrays.
    trace : Sum along diagonals.

    Examples
    --------
    >>> a = np.arange(4).reshape(2,2)
    >>> a
    array([[0, 1],
           [2, 3]])
    >>> a.diagonal()
    array([0, 3])
    >>> a.diagonal(1)
    array([1])

    A 3-D example:

    >>> a = np.arange(8).reshape(2,2,2); a
    array([[[0, 1],
            [2, 3]],
           [[4, 5],
            [6, 7]]])
    >>> a.diagonal(0,  # Main diagonals of two arrays created by skipping
    ...            0,  # across the outer(left)-most axis last and
    ...            1)  # the "middle" (row) axis first.
    array([[0, 6],
           [1, 7]])

    The sub-arrays whose main diagonals we just obtained; note that each
    corresponds to fixing the right-most (column) axis, and that the
    diagonals are "packed" in rows.

    >>> a[:,:,0]  # main diagonal is [0 6]
    array([[0, 2],
           [4, 6]])
    >>> a[:,:,1]  # main diagonal is [1 7]
    array([[1, 3],
           [5, 7]])

    The anti-diagonal can be obtained by reversing the order of elements
    using either `numpy.flipud` or `numpy.fliplr`.

    >>> a = np.arange(9).reshape(3, 3)
    >>> a
    array([[0, 1, 2],
           [3, 4, 5],
           [6, 7, 8]])
    >>> np.fliplr(a).diagonal()  # Horizontal flip
    array([2, 4, 6])
    >>> np.flipud(a).diagonal()  # Vertical flip
    array([6, 4, 2])

    Note that the order in which the diagonal is retrieved varies depending
    on the flip function.